I think it just takes a while to get used to riding a lot. When I started back at the age of 48 I recall my legs were continuously sore. I was always massaging with rolling pins and various devices. Now, 4 yrs later, it is much more difficult to get my legs sore. Unless I foolishly decide to hike up and down the local fitness climb (800m elev in 1.8km). I did the hike on Wed and my legs are still sore.